Hybridization of plants is recognized as an important process for producing offspring having a combination of the desirable traits of the parent plants. The resulting hybrid offspring often have the ability to outperform the parents in different traits, such as in yield, adaptability to environmental changes, and disease resistance. This ability is called "heterosis" or "hybrid vigor". As a result, hybridization has been used extensively for improving major crops, such as corn, sugarbeet and sunflower. For a number of reasons, primarily related to the fact that most plants are capable of undergoing both self-pollination and cross-pollination, the controlled cross-pollination of plants without significant self-pollination, to produce a harvest of hybrid seeds, has been difficult to achieve on a commercial scale.
In nature, the vast majority of crop plants produce male and female reproductive organs on the same plant, usually close proximity to one another in the same flower. This favors self-pollination. Some plants, however, are exceptions as a result of the particular morphology of their reproductive organs which favors cross-pollination. These plants produce hybrid offspring with improved vigor and adaptability. One such morphology in Cannabis ssp. (hemp) involves male and female reproduction organs on separate plants. Another such morphology in Zea mays (corn) involves male and female reproductive organs on different parts of the same plant. Another such morphology in Elaeis guineensis (oilpalm) involves male and fertile female gametes which become fertile at different times in the plant's development.
Some other plant species, such as Ananas comosus (pineapple), favor cross-pollination through the particular physiology of their reproductive organs. Such plants have developed a so-called "self-incompatibility system" whereby the pollen of one plant is not able to fertilize the female gamete of the same plant or of another plant with the same genotype.
Some other plant species favor cross-pollination by naturally displaying the so-called genomic characteristic of "male sterility". By this characteristic, the plants' anthers degenerate before pollen, produced by the anthers, reach maturity. See: "Male-Sterility in Higher Plants", M. L. H. Kaul, 1987, in: Monographs on Theoretical and Applied Genetics 10, Edit. Springer Verlag. Such a natural male-sterility characteristic is believed to result from a wide range of natural mutations, most often involving recessive deficiencies, and this characteristic can not easily be maintained in plant species that predominantly self-pollinate, since under natural conditions, no seeds will be produced.
There are four main types of male sterility observed in nature. All four types of male sterility are used in commercial breeding programs to ensure that there is cross-pollination to produce hybrid seed for crops such as corn, sugarbeet, oilseed rape and sunflower.
One type of male sterility is nuclear encoded and is believed to be inherited as a recessive allele. For breeding purposes, a recessive male-sterile parent plant is maintained by crossing it with a heterozygous male-fertile plant that also includes the recessive male-sterility allele, so that the offspring are 50% recessive male-sterile plants. The other 50% are male-fertile plants that have to be rogued out in outcrossing programs which can only be done efficiently if the recessive male-sterility allele is segregated together with a selectable or screenable marker. In U.S. Pat. No. 4,727,219, a procedure is described for the use of recessive male sterility for the production of hybrid maize.
A second type of male sterility is nuclear encoded but inherited as a dominant allele. An advantage of dominant male sterile plants, as compared to recessive male sterile plants, is that the dominant male-sterile plants can be maintained through crossing with a male-fertile plant, to produce offspring that are 50% dominant male-sterile plants. The usefulness of this dominant nuclear male-sterile plant is, however, limited because its dominant male-sterility allele is in most cases not tightly linked (i.e., within the same genetic locus) to a selectable or screenable marker.
A third type of male sterility is cytoplasmatically encoded. In most cases, the cytoplasmic code is in the mitochondrial genome of the plant, and only in a few cases is the code in the chloroplast genome of the plant. The inheritance of cytoplasmatically encoded male sterility does not follow Mendelian rules but rather depends on cytoptasmic factors. The offspring obtained from crosses between cytoplasmic male-sterile plants and male-fertile plants all carry the cytoplasmic male-sterility gene and are therefore sterile. As a result, the offspring of plants of this type are only of commercial value if the economic product of the offspring is not for use as seed but rather for plants such as ornamentals and sugarbeet.
A fourth type of male sterility is the result of a combination of both nuclear encoded male sterility and cytoplasmatically encoded male sterility. The male sterility-inducing nuclear alleles are usually recessive, and only plants that contain the male-sterility cytoplasmic allele and that are homozygous for the male sterility-inducing nuclear allele are phenotypically male sterile. In this type of plant, corresponding dominant male fertility-inducing alleles or "restorers of fertility", produce a male-fertile phenotype. As a result, the male-sterile offspring of this type of plant can be made male-fertile by pollinating the male-sterile plants with pollen containing the restorers of fertility. As a result, the offspring of plants of this type are of commercial value where the economic product is seed, that is for plants such as corn, sorghum and sunflower.
Typically, hybrid seed production has been accomplished by the large scale planting of cytoplasmic male-sterile plants and male-fertile plants and by somehow (e.g., with a distinctive marker) preventing the resulting hybrid seeds from becoming mixed with non-hybrid seeds. According to U.S. Pat. No. 3,842,538, hybrid seeds are tediously separated from non-hybrid seeds on the basis of color. According to U.S. Pat. No. 4,351,130, the problem of separating hybrid seeds from non-hybrid seeds is avoided by using short male-sterile plants and tall male-fertile plants and then destroying the tall male-fertile plants after pollination. According to U.S. Pat. Nos. 4,658,085, 4,517,763 and 4,658,084, cytoplasmic male-sterile plants are provided with a herbicide tolerance absent from the male-fertile plants which are destroyed with the herbicide after pollination. According to U.S. Pat. No. 4,305,225, male-sterile rice plants are sprayed with a growth hormone (e.g., gibberellin) in order to cause fuller emergence of flower-bearing panicles from rice leaf sheaths, thereby increasing the ability of the flowers to receive pollen from male-fertile plants.
In all such processes for producing hybrid seeds from male-sterile plants, ways have been sought for simply and inexpensively obtaining on a commercial scale: 1) high hybrid seed production from each male-sterile plant; 2) a hybrid seed population that results almost exclusively from pollen of male-fertile plants and eggs of male-sterile plants and is substantially free of non-hybrid seeds from male-fertile plants; 3) easy production of both the male-sterile and male-fertile plants; and 4) the virtually complete removal or destruction of either the male-fertile plants after they have pollinated the male-sterile plants or the selective separation of non-hybrid seeds, produced by the male-fertile plants, from the hybrid seeds produced by the male-sterile plants.